Desert regions, isolated islands, developing countries, industrial areas that require a large amount of fresh water, and so on, are examples of many regions in which a serious water shortage occurs chronically or seasonally as there is no fresh water source such as a river, a lake, or the like, and thus, a sufficient fresh water source is not available, or it is impossible to rely on rainwater due to climate characteristics. In parts of such regions, fresh water (drinking water) is produced by treating seawater, which is present in large quantities. As a method of producing fresh water, a method of filtering fresh water by passing seawater through a reverse osmosis membrane (a RO membrane) such as a hollow fiber membrane, a spiral membrane, or the like may be used.
Meanwhile, when fresh water is produced from seawater, the seawater needs to be pretreated before being passed through the reverse osmosis membrane. In the pretreatment, in general, in order to prevent biofouling by algae, shellfish, and so on, of a water intake port, a pipeline or a waterway configured to collect seawater, an oxidizing agent such as hypochlorous acid or the like is added. In addition, an inorganic coagulant such as ferric chloride or the like, or a polymer coagulant such as PAC or the like is added to an aggregate-suspended solid such as particulates, colloids, or the like in seawater, and filtered through a sand filter or a cartridge filter apparatus (CF) to remove the suspended solid in the seawater.
When a chlorine-based oxidizing agent remains, oxidation of the reverse osmosis membrane occurs, and thus, a decrease in membrane performance of the reverse osmosis membrane occurs. For this reason, before the seawater penetrates through the reverse osmosis membrane, a reducing agent such as sodium bisulfate (SBS) or the like is added while measuring an oxidation-reduction potential, to perform neutralization (for example, see Patent Literature 1).
When such pretreatment of seawater is performed, it is possible to prevent biofouling of the water intake port, the pipeline, the waterway, and so on, that are configured to collect the seawater, and early drop (fouling) of a membrane performance of the reverse osmosis membrane due to organism proliferation or silting of the suspended solid, and to efficiently and economically produce fresh water.